drop weight testing rig analysis and...

Pertanika J. Sci. & Techno\. Supplement 12(2): 159 - 175 (2004)ISSN: 0128-7680

© Universiti Putra Malaysia Press

Drop Weight Testing Rig Analysis and Design

I'T.M. Shuaeib, 1,2,·S.V. Wong, 1,2J{,S. Tan, 1,2A. M. S. Hamouda!Radin S. Radin Umar & 1,2M.M.H. Megat Ahmad

IDepartment of Mechanical & Manufacturing Engineering, Universiti Putra Malaysia,43400 UPM, Serdang, Selangor, Malaysia

2Road Safety Research Centre, Universiti Putra Malaysia,43400 UPM, Serdang, Selangor, Malaysia

E-mail: [email protected]

ABSTRACT

Crashworthiness studies are becoming increasingly important in mechanical design,particularly with the new advancement of the computer simulation codes. These studiesgenerally require material and prototype testing for both modelling and validation. Largepercentages of these studies lie on the limits of medium strain rate, which could beachieved by a drop weight test rig. Therefore, the drop weight test rig becomes anessential tool for such research activities besides the universal quasi-static testing machines.This paper is devoted to the analysis and design of the drop weight impact-testing rig.First, the different aspects of the mechanical design such as the propulsion, guidance,and frame layout, foundation and energy aspects are presented and discussed. Then, thebasic types of data retrieval and analysis systems applicable for drop weight impact testingmachines are presented and discussed. Data retrieval components considered in thisstudy include the sensors for load, acceleration, and velocity measurements, imageacquisition including high-speed cameras and PC-based image acquisition system, anddata acquisition including oscilloscope or PC-based data acquisition system which utilizesan AID card and application software for visualizing and analyzing of the results. At theend of this article the designed and constructed test machine is presented as a case study.

Keywords: Drop weight, impact. strain rate, low velocity, crashworthiness, helmet

INTRODUCTION

In this article the drop weight design requirements in general are presented. However,more emphasis is given to crashworthiness testing requirements or similar application.This needs a clarification here for distinguishing from other drop weight applicationssuch as that for metal forming.

Therefore, the basic requirements and the general guidelines for the design of dropweight testing rig will be presented and discussed in this article and case study of dropweight test rig which is designed and constructed for helmet testing and similarapplications will be briefly described.

In general, if a drop weight testing rig is required by a crashworthiness laboratory,there would be two ways to fulfill this requirement. The first is to purchase a generalpurpose prefabricated testing rig from specialized suppliers such as Dynatup orQUALTITEST (see references), and the second is to carry out the design and fabricationin-house. The first alternative is more reliable, as performance is usually guaranteed bysuppliers, but is expensive and working space is limited. The second alternative iseconomical and can be made with a large degree of flexibility, but professionalpersonnel especially in electronic equipment installation and commissioning is a musLIn fact, according to the authors' knowledge, there is no review or design article, which

* CtJrnsponding autlwr

F.M. Shuaeib, S.Y. Wong, KS. Tan, A.M.S. Hamouda, RS. Radin Umar & M.M.H. Megat Ahmad

is directly devoted to this issue. In a survey on the drop weight rigs available in relatedresearch works, it was found that various configurations and designs were used forcrashworthiness studies (Clements et aL 1988; Combet et al. 1998; Volt et al. 1998; Zho1998; Broutman and Rottem 1975; Colakoglu and Yildirius 1993; Lee et al. 1998; Amburet al. 1995, Kowalski et al. 1999; azar et al. 1996). This could give an impression that inmost of those rigs the second choice, which is the in-house built rig, might have beenemployed due to the reasons mentioned.

DESIGN SPECIFlCATION & REQUIREMENTS

Basic Mechanical Requirments

Drop weight tester, in contrast to other impact testing apparatus can be made in varietyof ways without appreciably altering the results of test results. The drop weight testershall be any suitable apparatus that conforms to the following requirements:• Permit accurate pre-positioning of the test specimen to assure free falling and

impact at the exact place and in the direction desired.• Permit accurate and convenient control of the height of the drop.• Permit a free unobstructed fall.• Provide a solid surface of concrete, or steel of sufficient mass to absorb all the

expected shocks without appreciable deformation or energy losses.

As mentioned previously, drop weight tester has not been standardized yet, therefore,several configurations with minor variations are found in the literature. These variationsdepend on the intended application. For example, a high-speed camera may be requiredfor test rig for academic study purposes but not for inspection (qualifying) requirements.

Macaulay (1985) highlighted most of the basic requirements for low velocity impactdrop weight tester. However, his information was presented in a general manner and didnot specify any particular application. For example, the range of low velocity impact,which could be produced by drop weight tester, was stated to be from 1 to 20 m/s. Thisdoes not mean that the total range would be applicable to all crashworthiness studies.The applicable range for a particular application is normally stated in related standard.For example, for helmet testing, most standard quote a velocity range between to 6-12m/s (BS 6658: 1985, MS 1:1996, and FMVSS 218). Therefore, some of Macaulay's (1985)data is presented in this article as a general guideline for drop weight testing machines.

Features of Basic Mechanical Components

The basic mechanical requirements are presented in the following paragraphs. It is tobe emphasized here that this article is not considered as a detailed design reference, buta basic guideline as some details will depend upon the exact application intended whichis helmet testing that may not be suitable for other applications.

Propulsion and Guidance

Two essentials for the success of impact tests are means of bringing a moving mass upto speed and a means of ensuring that the impact occurs at the right location and in thecorrect orientation. In general, accurate guidance is more important than accuratespeed, because its easier to make allowances for speed variation in a test which isotherwise correct than to compensate, for instance, for an event occurring partly out ofthe field of view of a camera. Some typical types of propulsion are shown in Fig. 1. Asshown, at low speeds, modified static test apparatus may be used. Gravity drop weight test

160 PertanikaJ. Sci. & Techno!. Supplement Va!. 12 No.2, 2004


---'Modified static test machines

l /,Ind()()( ,i9S :/'F,ee fatli"9 bombs

Gravity drop ----.t---------------,Mechanical propulsion~

Eltplosive CJUns---1

Direct eJtploSives-----1

Light <)as QUns----1

DC electric 'ail 9IJrls--1

10 102 103

Speed (m S-l)

Fig. 1: Types of propulsion (Macaulay 1985)

rigs provide a simple and repeatable means of propulsion at speeds of up to about 25mls (Macaulay 1985).

With small specimens used to test material properties, guidance is usually maintainedthroughout the test. Otherwise guidance might be removed once the moving mass is upto speed. Guidance can be achieved in various ways such as wire ropes with non-steelbushings, hardened steel, andlor stainless steel rods with rolling bearings, or structuralsteel with pulleys mounted on bearings (Hamouda and Hashmi 1998; Broutman andRottem 1975 and Colakoglu and \}ldirius 1993). It is to be emphasized at this stage thatany guidance shall have minimum friction forces acting on the drop mass.

Frame Layout

One of the interesting issues which face the designer is that various configurations arepossible for the frame skeleton of the rig. So, what is the most suitable frame layout?This might lead to another question, which is, what the existing designs look like andwhat their positive and negative features are. In other words, what can we learn from thI'available data on drop weight rigs, which is scattered on the low velocity impact 'lIhl'« tpublications. Of course, none gave complete details of their design, but by collectillg 1)11'

and pieces from different sources, reasonable figures can be combined, which togl'tltl'lwith basic structural mechanics knowledge, could be used to assist in design rulesdevelopment. Fig. 2 shows sketches of few layouts, which could be utilized. Thedifference between Figs. 2 a and b, is the position of the columns, whether on top of thefoundation or directly on to the ground floor. The choice will depend on the size of themachine and the degree of flexibility required (possibility of repositioning or removingthe foundation).

PertanikaJ. Sci. & Techno!. Supplement Vo!. 12 No.2, 2004 161

F.M. Shuaeib, S.Y. Wong, K.S. Tan, A.M.S. Hamouda, R.S. Radin Umar & M.M.H. Megat Ahmad

(a) (b) (c)

Fig. 2: (a) Columns on faundation, (b) Columns on graund floor, (c) Columns on the back

Both layouts in Figs. 2 a and b could have two in plane or four columns as a boxframe depending on the intended application and rigidity requirements. However, Fig.2 c, which shows two columns at the back layout, is the least rigid and could be used forsmall to medium applications (large weight would create large bending stress on theguides and consequently high friction forces).

Foundation

The type and size of foundation depends upon the application requirement, particularlythe impacting mass and speed. In general, the main criterion is the mass of thefoundation. The materials could be steel or strongly reinforced concrete to withstandimpact loads. The other criterion that needs to be considered is the damping capabilityof the rig which can be almost fulfilled by providing a thick mat of rubber or similarmaterial at the bottom of the foundation. This layer would absorb the shock and preventany reflected stress waves from interference with the test results. Regarding the maincriterion which is the foundation mass, certain application standards which use dropweight machines such as helmet testing standards generally specify the minimumfoundation mass required (BS 6658: 1985, MS 1:1996 and FMVSS 218).

Strnctural Design Checks

In order to have confidence on the frame strength, some structural checks are necessary.First, the design load is determined based on the intended application. Then, the freebody diagram can be drawn and the forces on each member determined and analysed.The shear force and bending moment diagrams are then drawn for each link. Crosssections are estimated based on simple mechanics theories for bending, deflection, andcolumn buckling. The simplified procedures can be used and there is no need forcomplicated calculations, particularly for floor-mounted frames where the case could beconsidered as a completely static loading design. Figs. 3 a and b show an exaggeratedsketch to provide better visualization of the anticipated deformation path. Such proceduresare well documented in the literature (Ambrose 1997; Cero and Cown 1976;Brockenbrough and Merritt 1999).

162 PertanikaJ. Sci. & Techno!. Supplement Vo!. 12 0.2,2004


Applied loadBending

-~-

Buckling;1

a) Front view of frameloading

b) Expected deformationpath

Fig. 3: Anticipated deftmlUltion shape fur one view of the frame

Energy Consideration

In general a gravity drop weight consists of a steel base plate assembly, a ram that holdsthe striker, supporting columns, which guide the falling weight (ram and the striker)and a device to raise the ram and the striker to the desired height. The kinetic energyof the falling weight depends on its weight and the height of the fall (Colakoglu andYildirius 1993).

The energy is mainly dissipated in the following ways:• Deformation of the specimen• Rebound of the striker assembly.• Elastic deformation of the drop weight machine.

The first two ways are to a great extent dependent on the deformability of thematerials constituting the specimen. The energy dissipated in deforming the specimenwould be termed the crash energy E,

The energy of rebound depends upon the coefficient of restitution between the twosurfaces. If m j and VI are the mass and the rebound velocity of the striker assemblyrespectively, the rebound energy E, is given by:

(1)

Some of the energy is lost in the guidance due to opposing frictional forces (E ). Theg

machine frame absorbs a certain amount of energy due to vibration of different parts.This is because of the transfer of momentum from the striker. The test specimen willalso absorb certain amounts of energy in the form of heat. Grouping all these lossesunder the heading of the transmitted energy Ef total energy, E, created by dropping thestriker can be expressed as:

E=E + E + E + Eerg t (2)

However, this energy is equal to the initial potential energy of the striker assemblyat its falling height h.

PertanikaJ. Sci. & Technol. Supplement Vol. 12 No.2, 2004

(3)

163

F.M. Shuaeib, S.V. Wong, KS. Tan, A.M.S. Hamouda, R.S. Radin Umar & M.M.H. Megat Ahmad

The test specimen undergoes a plastic deformation in a non-elastic collision betweenthe striker and the test specimen. The ratio of energy consumed in deforming the testspecimen to the total potential energy of the striker is defined as the blow efficiency (TJ).

Ef

can be directly calculated ,if TJ b and E are known.The blow efficiency, defined in the above equation can be calculated from the

following equation:

where "e "is the coefficient of restitution and defined as

(6)

and ~ and ~ are the weight of the striker assembly (falling weight) and the weight ofthe steel base plate assembly respectively. ~ and ~ are after impact velocities of thestriker and the steel plate assembly respectively. Ya is the initial impact velocity of thestriker. In many cases, the rebound velocities ~ and ~ are small compared with theimpact velocity of the striker Ya. Therefore, the factor "e" approaches zero. Then the blowefficiency becomes

(7)

When this ratio is determined for a particular design, then E1

can be calculated fromEquations (I) and (7).

It could be observed that for a more efficient impact test the striker assembly massshould be minimized. This approach was considered in the drop weight designed in thisresearch program by increasing the height to the maximum feasible (see next section).• The Impact Velocity: The impact velocity is given by

Vo=·../2ah (8)

where h is the height of the drop, "a" is the acceleration of the striker• Friction Effects: If R is the frictional force and P is the constant vertical force (forgravity drop P = 0), then the acceleration of the striker is equal to

WI +P-Ra= (9)

ml

where m\ is the mass of the striker.A5 mentioned previously for gravity drop weight tester, no external driving force is

applied on the striker (i.e. P = 0), therefore equation (9) takes the form

164

Ra=g-

ml

PertanikaJ. Sci. & Techno\. Supplement Vo\. 12 0.2,2004

(10)


It is clear that in gravity drop weight testers, the acceleration of the striker is slightlyless than the gravitational acceleration (a < g), due to frictional losses. To have amaximum impact speed of the striker for a given height, the frictional force (R) has tobe minimum.

DATA RETRIEVAL

The importance of properly selected data retrieval systems for drop weight testingmachines need not be emphasized here. Much care should be taken in the selectionprocess to obtain meaningful results.

Various data retrieval components in use for drop weight testing rigs are scatteredin the literature, which are either identical or slightly different (Collombet et al. 1998;Volt et at. 1998; Zho 1998; Broutrnan and Rottem 1975; Winkel and Adams 1985;Gilchrist and Mills 1996; Gilchrist and Mills 1994). This part explores this subject andprovides some useful guidelines in selecting drop weight data retrieval systems ingeneral.

In this section data retrieval system means all equipment/ instrumentations used toretrieve and analyse data during an impact testing. This includes the following:• Electronic instrumentations (Transducers)• Image acquisition System (Photography equipment).• Electronic data acquisition system.

Due to the wide range of applications of these instruments, only the essentialpractical issues to the drop weight impact testing machines have been considered here.Electronic measurements and data acquisition theories are well documented in relatedtexts and periodicals (Coombs 1999).

Transducers (Semars)

Transducers are the sensing elements, which are used to measure the impact responserelated parameters such as load, acceleration and velocity. The three transducers, whichare selected for the drop weight impact testing machine under this study, are theaccelerometers, load cells, and photo sensors. This selection has been made based on dropweight rigs instrumentation found in the literature for crashworthiness studies andconsidering helmet testing performance requirements (Broutrnan and Rottem 1975;Winkel and Adams 1985; Dias 1999; MS:l 1996).

In general, the main advantages of electronic transducers are the provision ofcontinuous records in a form suitable for automatic processing. The disadvantages arethat a separate channel is needed for each item recorded, and that transducers are easilydamaged and they can modify the behaviour of the test specimen (Macaulay 1985). Forboth accelerometers and load cells the piezoelectric types are the best choice for shockresistance capability (Gilchrist and Mills 1996; Gilchrist and Mills 1994; Dias 1999; BS6658: 1985; MS 1:1996; and FMVSS 218). Details on piezoelectric principles will not becovered here as such data can be found in relevant textbooks or recognized companyproduct catalogues such as Kistler and Instron (Dias 1999; Kistler Product Catalogue2001). For preliminary selections, a margin is needed for the expected highest accelerationor load, taking into consideration that a very large gap between the expected test valuesand the instrument range may increase the percentage of the ± error, particularly for theload cell. Photo sensors are adopted in drop weight testing rigs as means for measuringvelocity between two vertical points or just before impact. Furthermore, they can beeffectively utilized for data capture via external triggering connection in the data

PertanikaJ. Sci. & Techno!. Supplement Vol. 12 No.2, 2004 165

F.M. Shuaeib, S.v. Wong, K.S. Tan, A.M.S. Hamouda, RS. Radin Umar & M.M.H. Megat Ahmad

acquisition card. These sensors are specialized equipment and sold by few recognizedcompanies. They can be of through beam type or reflective type. Generally the throughbeam is utilized for drop weight testing rigs (Gilchrist and Mills 1996; Gilchrist and Mills1994; Dias 1999 and KEYENCE product Catalogue 2001).

Image Acquisition System (Photography)

Photographs record only the'displacements but they can do this in great detail. All typesof impact can be recorded and taking of photographs usually has no effect on the testitself. A major use of photographs is to allow impacts to be studied qualitatively at leisureand this can give considerable insights to pattern of behavior without further analysis(Macaulay 1985). Recently, photographic apparatus, particularly high-speed photography,is increasingly being used in drop weight testing machines.

Therefore, it was decided to include the photographic equipment as a maincomponent for the drop weight-testing machine in this study.

Two methods which can perform this task are now in the market. The first is by usinga stand-alone high-speed digital camera system, which is normally positioned on tripodsat suitable locations as shown in Fig. 4, (EASTMAN KODAK COMPANY). The imagesand movie files can be transferred to the PC after the test is completed. Therefore, thestorage capacity of the camera is an important factor to be considered. The secondmethod is by using the computer-based image acquisition techniques where the digitalcamera is connected to a standard PC through image acquisition card (IMAQ). Then byusing an image visualization and analysis software, such as IMAQVISIO plug-inprogram to labVIEW developed by National Instruments, the impact test photographycan be processed and analyzed. An example of such a system is shown schematically inFig. 5. Also, Fig. 6 shows a PC-based image acquisition system using Sony digital cameraas an integral system assembled by ational Instruments as a complete image acquisitionpackage ( ational Instruments Catalogue 2001).

Analogue cameras can still be used as a stand alone or as PC-based image acquisitionsystem but generally they have low frames per second capability (less than 35 fps).Therefore, for crashworthiness studies digital cameras have replaced the analoguecameras due to the superior performance such as the high frame rate, high spatialresolution, and high contrast.

In general, the most important parameter that has to be precisely specified forproper photographic performance is the frame rate per second for the digital camera.

166

Fig. 4: Stand alone high speed digital camera system (EASTMAN KODAK COMPANY)

PertanikaJ. Sci. & Techno!. Supplement Vo!. 12 No.2, 2004


Cabling

Softwdr

Fig. 5: Typical PC-based image acquisition system (National Instruments Catalogue 2001)

Fig. 6: An integrated image acquisition system (National Instruments Catalogue 2001)

The range of frame per second for various photographic equipment is shown in Fig.7 (Macaulay 1985). The quality of the picture increases as the framing time increases,but the cost also increases sharply. Though the figure may not be recent, it still gives anidea about the significance of the framing parameter.

Therefore, it is clear that the experience is essential in deciding on the frames persecond (fps) required for certain applications, as any mistake may lead to redundantcost. Broutman and Rottem (1975) used 6000 frames per second for drop weight testingmachines to study the impact strength of fibre composite materials. Therefore, this valuecan be considered as suitable with a minimum of about 1000 fps. For better visualizationperformance on high speed impacts the frames per second can be increased up to10,000.

Finally, it is worth mentioning here that the need for high speed camera system fora particular application must be carefully evaluated, as in certain circumstances it maynot be necessary or it does not provide vital information to justify the high capital cost.

PertanikaJ. Sci. & Techno!. Supplement Vo!. 12 0.2,2004 167

-----•• Inlermittent motion

F.M. Shuaeib, S.v. Wong, K.S. Tan, AM.S. Hamouda, RS. Radin Vmar & M.M.H. Megat Ahmad

----. Conventional cameras and lighling

-----•• Video

'"~~ Rotating prism ••---.5.

Rotating mirror ••-----..

Electronic camerllS ..·---------..

Lllsers ••---..

Maximum limit ....\

Equivalent for accelerometer • Theoretical limit-----.l

10 1010

Frames per second

Kerr cell 1Flash radiograph ••---_

Spark shadowgraph,

Front tit flash ••-----Convent ionlllc.meres -+llnd tighting

Fig. 7: Comparisons of camera speeds in frames per second (Macaulay 1985)

Data Acquisition Systems

A data acquisition system is defined here as all the components behind the sensors inthe measuring loop. The function of this system is to handle the signal, performfiltration and tuning, and make the necessary visualization and analysis. The system canbe designed in different ways to achieve specific requirements. For drop weight impacttesting machines, the number of sensors are usually limited and specified. Therefore,the most applicable data acquisition systems can be divided into two methods/categories:• Method A: Using Oscilloscope.• Method B: Computer-based data acquisition systems (Virtual Instruments).

Both methods include signal-eonditioning components either as built-in or separatelyconnected. The signal conditioning components are usually filters and amplifiers. Theseunits are required to tune and filter out the unwanted noise and consequently allow formeaningful visualization of results. Filters and amplifier signal conditioning sometimescomes in a single unit, which includes both functions in a mUltipurpose unit or can bepurchased individually. Certain parameters need to be known in selecting the suitableunits including sensor output voltage.

However, as a useful hint, quite often well-known vendors will assist the purchaserin selecting the most suitable unit if sufficient details about the whole system, particularlythe sensors are provided. Data acquisition systems are described in the followingparagraphs.

• Method A (Using Oscilloscope)For this method, the features of the oscilloscope are the governing factor of the output

data quality, as more sophisticated digital oscilloscopes are now available in the market.These can handle large amounts of data and can have large numbers of channels, and

168 PertanikaJ. Sci. & Techno!' Supplement Vo!. 12 No.2, 2004


signals can be tuned very accurately. Also the results can be printed or stored in a floppydiskette or other hard drives, which can be transferred to a standard program like Excel.However, oscilloscopes, which are suitable for impact testing with sufficient features aregenerally expensive and have no means for upgrading. The degree of flexibility islimited with the available features and no modification/ programming can be made tothe output results. Fig. 8 shows a typical digital oscilloscope.

Fig. 8: Digital oscilloscope with enhanced features such as floppy diskette storage facility (FD)and external trigging (Yokogawa product catalogue 2001)

• Method B (Computer-based data acquisition systems)This method is termed computer-based data acquisition system, which basically consistsof the components shown in Fig.9 (excluding the transducers). Generally, in thistechnique, the sensor analogue signal (Low voltage electric signal) is conditioned first,then converted to digital signal through A/D cards (boards). Then, the applicationsoftware will be used to visualize and analyse the results (Stahlin 1999; Corcon 1999;Stahlin and Christ 1999; Kwock 1999; National Instruments Catalogue 2001).

measurement and nnalysisHardware

Signal conditioningTransducer Software

Fig. 9: Simple PC-based data acquisition system (National instruments Catalogue 2001)


F.M. Shuaeib, S.Y. Wong, K.S. Tan, AM.S. Hamouda, RS. Radin Umar & M.M.H. Megat Ahmad

After AID conversion, an application program is generally required for visualizingand analysing the results. These two components are described separately in the nextparagraphs.

The AID Conversion Card: This is the most critical part of the data acquisition system,which needs to be carefully selected. It can be considered the heart of the system. AID cards can be for analogue I/O, digital I/O, or for timers and counters. Also, somecards are designed as multifunction cards where all previous tasks could be performed.The latter are the most suitable for drop weight-testing machines as they have more thanone type of data channel. There are several parameters, which have to be specifiedbefore an AID card can be properly selected. Among those are: the type of PC platformused, number of channels, sampling rate, analogue bandwidth, resolution, and triggeringmode. AID boards for various platforms such as Desktop (PCI & ISA) , Laptop (PCMCIA),Industrial PC (PXI) , or Workstation (VXI) are now widely available. Therefore, accordingto the type of computer (platform) allocated to the drop weight machine, the propercard will be selected. However, it is to be noted that each type of platform has itsadvantages and disadvantages. For example, desktop is inexpensive, readily available,and the latest technology is obtainable, but rack mounting is difficult and few expansionslots are available, while PXI computers are well suited for rack mounting, have moreslots, the same software as desktop, back panel timing and triggering, but it is moreexpensive. The second parameter (number of channels) is selected according to thenumber of instruments expected with some reserve for future expansion. The thirdparameter, which is the sampling rate, is also critical. If it is not specified properly aliasing of the results would occur and the whole system result will be useless ormisleading. Fig. 10 shows this effect more clearly. Therefore, it is very essential tocorrectly estimate the number of samples, which can represent the required output inthe test time domain. This requires some experience on the estimated durationmechanism of the impact events under consideration. However, theoretical equationscan be obtained in data acquisition references, which would assist in this estimation(Corcon 1999; Vilbiss 1999; ational Instruments Catalogue 2001).

The analogue bandwidth is also an essential parameter, which can be overlooked inthe selection process or may not be provided by the AID card vendors. In general,analogue bandwidth describes the frequency range (in hertz) in which a signal can beaccurately digitised. This limitation is determined by the frequency of the input path.Input signals with frequencies above this bandwidth will result in loss of amplitu~e and

Adequately Sampled

Aliased Due to UndersamplingFig. 10: Effects oj sampling rate on the output signal (National instruments Catalogue 2001)

170 PertanikaJ. Sci. & Techno!. Supplement Vo!. 12 No.2, 2004


phase information. The effect of this parameter is shown in Fig. 11. The most efficientway to overcome such a problem is by specifying the sensor data clearly to the AID cardvendor who in turn will advice if there are any restrictions. In Fig. 11, the output signalis only up to the card limit (National Instruments Catalogue 2001)

.2V- ~---------.------.-.. --

.., I• b c

OV··- ---- .• --- -.--------1'---+

-lV·--,----·----- -d--2V .. -.------.---- ------------

.1f.2VOV-1/2 V

Input Signal Instrument Measured Signal

Effects of Insufficient BandwidthFig. 11: Effect of bandwidth on the measured signal (National Instruments Catalogue 2001)

The last parameter is the triggering mode. In general, triggering means the processof determining the exact instant in time to begin the signal capture. This item is, in fact,a challenging parameter and could be considered as a main factor in comparingdifferent AID cards in the purchasing stage. AID cards have various triggering provisionsand these generally determine their final price. Fig. 12, shows one of the sophisticatedAID cards, which has various triggering modes. The specification of the card is usuallydefined if the triggering mode is analogue (external), digital (internal) or softwarecontrol. The software control mode of triggering is usually done through the dataacquisition software. This mode of triggering is often limited mainly by the computertype and speed. However, an AID card which has an external triggering provision isusually more expensive, but it is quite beneficial for drop weight testing machines.

Fig. 12: Front view of typical computer-based data acquisition board with external triggering(National Instruments Catalogue 2001)

PertanikaJ. Sci. & Techno!. Supplement Vol. 12 No.2, 2004 171

F.M. Shuaeib, S.Y. Wong, K.S. Tan, AM.S. Hamouda, RS. Radin Umar & M.M.H. Megat Ahmad

The Application Software: The application software function is to visualize and analyse thedata acquired. This can be achieved by different ways. For researchers who have no timeand knowledge on advanced programming languages such as C++ or Visual Basic, usingany of the available graphically-oriented data acquisition and analysis software isrecommended. Three competitive data acquisition programs are most suitable in themarket which can be utilized.

These are namely: VEE from Hewlett Packard (Stahlin and Christ 1999), labVlEW,and DASYLab. The last two programs are ational Instrument products and would beconsidered in our case due to the availability of many users, and Notational Instrumentscompany technical support availability in South-east Asian countries, particularly inMalaysia. Considering labVlEW and DASYLab programs, the former is more flexible andhas more features such as programming ability and 3-D plots while the latter is meantmainly for data acquisition purposes, easier to use but with less flexibility, particularly ifprogramming is required The choice will depend on the applications complexity andextent of the testing required. However, if the requirement is only for simple dataacquisition and the data acquisition components are within the available DASYLabBlocks/ Modules, then it may be the best choice. Otherwise, labVlEW would be moreappropriate, particularly if the future application will require more facilities such asimage acquisition system or similar application. The important point, which needs to beconsidered, is that the type of A/D card must be supported by the selected program.Also, some recognized companies, such as DEWETRO ,usually develop their own dataacquisition programs to work with their cards. Such programs usually have limitedcapability and the use is limited to that particular brand. However, it is noted that suchcompanies generally produce cards which work well with advanced programs such aslabVlEW or DASYlab, but this usually needs to be clearly mentioned in the A/D cardspecification before purchasing.

Recently, National Instruments developed a family of data acquisition programs withdifferent capabilities and labVlEW is one of them. These programs vary in features andtheir selection will depend upon the application, their complexity starting from simplenon-programmable data acquisition only and ending with programming and dataacquisition. The reader may refer to ational Instruments' web site for further details.It is noted that labVlEW seems to be dominating the National Instrument market, whichmight be due to the degree of flexibility and support from the company.

Other programs are available from different companies which have intermediatecomplexity and their use sometimes requires reasonable programming skills.

CASE STUDY

Based on the previous guidelines, a drop weight machine is designed and constructedwith the following details:

The overall height of the rig is 4 meters. The box beam frame has been selected asframe layout with the size of the square cross-section beam equal to 5 inches, asupporting intermediate box beam structure of 4 inch size has been installed at 2 metersheight to provide more strength against buckling. A simple manual wire rope and pulleylifting mechanism has been installed. A motorized system would be installed at a laterdate. A heavy-duty steel flange with recession for washer type load cell has beenfabricated. The foundation mass is one tone which is considered sufficient for thehelmet-testing intended application and also considered suitable for similar applicationssuch as material testing. However, for higher elevation requirements or heavier loads,the foundation could be removed and the steel flange could be anchored directly to thefloor. Therefore, the impact could be sustained by the more rigid reinforced concrete

172 PertanikaJ. Sci. & Techno\. Supplement Vo\. 12 No.2, 2004


floor or possibly different design foundation replacement. A 25 mm rubber matt hasbeen introduced between the floor and the foundation for initial and reflected shockwave absorption. A one-inch normal steel bar with guided rubber pulleys mounted onfriction-less ball bearing has been provided. Fig. 13 shows the front views of the dropweight that has been constructed. In fact, this pulley system will have some frictionaleffects on the striker velocity, but as it is intended to measure the velocity with photocelljust before the impact, minor losses could be neglected.

The details of the selected data retrieval system are presented in Table 1. Due to costconstraint, high-speed camera with 10, 00 fps is under bidding and not yet purchased.

TABLE 1Selected data retrieval components

Item

Sensors

A/D Cards(Two brands forreserve and flexibility)

Application Software

Instrument Details

Load Cell washer TypeShock accelerometer

Photoelectric Sensors

i) -Low-CostMulti-function Cardii) -Multi-function card

LabVIEW

Specification

100kN washer type, 9102APezotron low impedance,8742ATwo Thru- beam Type

- PCI- 6023E200 ks/s-ICT-DAS, No: PCI 1800H-nda 330 ks/sFull developmentSystem for Windows 2000/NT/Me/9x, Version 6

Brand

KistlerKistler

KEYE CESensorsNational

InstrumentsSDC

ationalInstruments

CONCLUSION

From the previous drop weight analysis and conceptual design, we can conclude thefollowing points:• For crashworthiness studies, particularly those dealing with prototype testing, it is

better to have an in-house fabricated drop weight machine which is built accordingto the required degree of flexibility, work space and rigidity rather than purchasingan expensive limited workspace drop testing rig.

• The ground fixed box frame layout is the most suitable, from the rigidity andstability point of view, but is less flexible.

• For more efficient impact the mass of the striker should be minimized and otherparameters such as the height could be increased to achieve the same impact energy.

• A simple economical guiding system could be designed if the velocity could bemeasured just before impact by other means, such as photocells.

• The selection of transducers depends on the exact application. However, pezoelectric accelerometers and load cells are the choice for drop weight machines.

• Photography is becoming essential for drop weight machines. Either·a stand aloneof PC-based systems could be utilized.

• Data acquisition could be achieved using digital Oscilloscope or PC-based dataacquisition system where AID board is utilized with application software.


F.M. Shuaeib, S.Y. Wong, KS. Tan, A.M.S. Hamouda, RS. Radin Umar & M.M.H. Megat Ahmad

Therefore, this article has provided some useful criteria and worked example fordrop weight testing machine that is suitable for crashworthiness studies. However, someminor points could not be completely covered in this article to avoid unnecessarycomplications.

ACKNOWLEDGEMENTS

The financial support by the Road Safety Research Center is greatly appreciated.

REFERENCES

AMBROSE, J. 1997. Simplified Design ofSteel Structures. 7th edition. Parker/Ambrose Series of SimplifiedDesign Guides.

AMBUR, D. R., C. B. PRUSUD, W. A. WATERS, R. W. STOCKUM and M. A. WALER. 1995. Internally damped,self-arresting vertical drop weight impact testing apparatus. US Patent No. 5497649.

BRJTISH STANDARDS, BS 6658: 1985. Specification for protective helmets for vehicle users.

BROCKENBROUGH, R. L. and F. S. MERRITT. 1999. Structural Steel Designer's Handbook. 3'" edition.McGraw-Hill Handbook.

BROUTMAN, L. J. and A. ROTTEM. 1975. Impact strength of fiber composites materials. ASTM STP 568.

CLE.I\iENTS, E. W.,J. R. SULLNAN and I. UNIGNESS. 1988. Shock testing machines. In Shock and VibrationHandbook, ed. C.M. Haris.

COlAKOGLU, A. and R. O. YILDIRIUS. 1993. Design and construction of gravity drop hammer withsuspended guides. In Modelling Measurement and Contro~ p. 19-32. ASME Press.

COlLOMBET, F., X. LALBIN and J. L. UTAIllADE. 1998. Damage prediction of laminated compositesunder heavy mass-low velocity impact. In Key Engineering Materials, ed. J.K Kim and TX. Yu,141-143: 743-776.

CORCON, J. J. 1999. Analogue-to-digital converters. In Electronic Instruments Handbook, ed. Clyde F.Coombs JR. Third edition. Chapter 6.

DASYLab data acquisition software of DATALOG, a National Instruments Company available at theInternet at web site: htq>://www.dasytec.com.

DIAS, J. F. 1999. Transducers. In Electronic Instruments Handbook, ed. F. Clyde Coombs JR. Thirdedition. Chapter 5.

Federal Motor Vehicle Safety Standards (FMVSS) , No: 218, (49 CFR sec. 571.218), MotorcycleHelmets (2001).

GERO, J. S. and H. J. Cown. 1976. Design of Building Frames. London: Applied Science PublishersLTD.

GILCHRIST, A. and N. J. MIlls. 1994. Impact deformation of ABS and GRP motorcycle helmet shells.Plastics, Rubber, and Composites Processing and Applications 21: 141-150.

GILCHRIST, A. and N. J. MJus. 1996. Protection of the side of the head. Accident Analysis and Prevention28(4): 525-535.

HAMOUDA, A.M.S. and M.SJ. HAsHMI. 1998. Testing of composite materials at high rates of strain:advances and challenges. JMaterials Processing Technology (77): 327-336.

Instron brand, Dynatup Drop Weight general purpose test rigs, Available at the Internet at HomePage of: htq>://www.lnstron.comlimpactlindex.htrn.

174 PertanikaJ. Sci. & Technol. Supplement Vol. 12 0.2,2004


KEYENCE Sensors, Product Catalogue 2001, available at the Internet at http://world.keyence.com

Kistler Product Catalogue 2001, available at the Internet at http://www.kistler.com

KOWAlSKI, E., G. S. LoCKE, T. RUSSELL and T. KONIECZNY. 1999. Individual Component HeadformImpact Test Drive. US Patent No. 5922,937.

KwoCK, D. 1999. Virtual instrumentation. In Electronic Instruments Handbook., ed. Clyde Third edition.F. Coombs JR. Chapter 46.

LabVIEWof ational Instruments available at the Internet at web site http://www.ni.comllabview.

LEE,J. E. and W. G. Prrr. 1998. Accelerated impact testing apparatus. US Patent 0.5,739,411, Apr.14.

Malaysian Standards. 1996. MS: 1. Specification for protective helmets for vehicle users. SecondVersion.

Malaysian Standards. 1998. MS: 1, Specification for protective helmets for vehicle users. SecondVersion.

MACAUlAY, M. 1985. Introduction to Impact Engineering. Chapman and Hall.

ational Instruments measurement and automation Catalogue 2001, available at the Internet athome page of: http://ww.ni.com

AZAR, L. M. 1996. Drop weight type impact testing machine. US Patent No. 5,567,867.

QUALITEST Quality Instruments available at the Internet at http://www.qualitestinc.com/dropweight.htrn.

SOC Company Product catalogue, available at the Internet at http://www.sdc.com.br/Prod/Automacao/Placas/index.htrn.

STAHLIN, B. 1999. Introduction to electronic instruments and measurements. In Electronic InstrumentsHandbook., ed. Clyde F. Coombs JR. Third edition. Chapter One.

STMAN KODAK COMPANY, Motion Analysis Division. Product Catalogue 2001, available at theInternet at web site of: http://www.masdkodak.com.

VILBISS, A. J. D. 1999. Oscilloscopes. In Electronic Instruments Handbook., ed. Clyde F. Coombs JR.Third edition. Chapter Fourteen.

VOLT, A., E. KROON and G.L. ROCCA. 1998. Impact response of fiber metal laminates. In KeyEngineering Materials, ed. J.K. Kim and TX Yu, 141-143: 235-276.

WENCIL, J. D. and D. F. ADAMS. 1985. Instrumented drop weight impact testing of cross-ply andfabric. Composite (4): 268-278.

Yokogawa Product Catalogue 2001, available at the Internet at htt;p:l/www.yokogaea.com/trnl

ZHO, G. 1998. Damage resistance and tolerance in thick laminated composite plates subjected tolow-velocity impact. In Key Engineering Materials, ed. J.K. Kim and TX. Yu. 141-143: 305-334.

PertanikaJ. Sci. & Technol. Supplement Vol. 12 No.2, 2004 175

drop weight testing rig analysis and...

Documents